CA2171325A1 - Apparatus and method for locating cellular telephones and similar transmitters - Google Patents
Apparatus and method for locating cellular telephones and similar transmittersInfo
- Publication number
- CA2171325A1 CA2171325A1 CA002171325A CA2171325A CA2171325A1 CA 2171325 A1 CA2171325 A1 CA 2171325A1 CA 002171325 A CA002171325 A CA 002171325A CA 2171325 A CA2171325 A CA 2171325A CA 2171325 A1 CA2171325 A1 CA 2171325A1
- Authority
- CA
- Canada
- Prior art keywords
- tdoa
- channel
- data
- signals
- cell phone
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S1/00—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
- G01S1/02—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
- G01S1/08—Systems for determining direction or position line
- G01S1/20—Systems for determining direction or position line using a comparison of transit time of synchronised signals transmitted from non-directional antennas or antenna systems spaced apart, i.e. path-difference systems
- G01S1/24—Systems for determining direction or position line using a comparison of transit time of synchronised signals transmitted from non-directional antennas or antenna systems spaced apart, i.e. path-difference systems the synchronised signals being pulses or equivalent modulations on carrier waves and the transit times being compared by measuring the difference in arrival time of a significant part of the modulations, e.g. LORAN systems
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W64/00—Locating users or terminals or network equipment for network management purposes, e.g. mobility management
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
- G01S5/06—Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
- G01S5/12—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves by co-ordinating position lines of different shape, e.g. hyperbolic, circular, elliptical or radial
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M11/00—Telephonic communication systems specially adapted for combination with other electrical systems
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
- G01S5/04—Position of source determined by a plurality of spaced direction-finders
Abstract
The field of locating cellular telephones suffers from high multipath city environments which affect accuracy. This problem can be overcome by a method for determining location by the time difference of arrival, tdoa, of signals from a cellular telephone (21) or other transmitter at cellular based stations (12-14). The method (20) includes determining tdoas for a plurality of channels, averaging the tdoas from the channels, subtracting the average tdoa from the individual channel tdoas to determine residual channel tdoas and determining an overall residual tdoa from the residual channel tdoas. Applications include emergency calls and criminal in investigations.
Description
2~ 7132S
.
APPARAT~S AND h~ ~iOv FOR ~OCATING r~rr~T~ TEL~r~o~.c AND STMTr.~R TRANSh~ SKS
S Bach~Ou~d of the Invention Field of the Invention The present invention generally relates to the function of locating cellular telephones and similar mobile transmitters, and particularly, to a method and apparatus which use time-difference-of-arrival techniques and which take advantage of the multiple channels used by cellular telephones and similar transmitters for this purpose.
lS Statement of the Prior Art The proliferation of cellular telephones, or cell phones, and their technology and usage, has revealed various applications for the ability to locate cell phones. These applications include ~911~ calls, tourist and travel information, tracking of unauthorized cell phone usage and illegal activities, and the locating of commercial and government vehicles, to name a few. The basic cell phone systems can only determine the nearest cell phone base station, which locates the cell phone to 2S within 3 to 10 miles.
A goal of any such system for use with cellular phones is the use of as much existing e~uipment as possible or at least compatibility with existing e~uipment. Examples of this are use with unmodified cellular phQ~e and compatibility with existing cellular base stations.
A great deal of technology already exists concerning the tracking or locating of radio transmitters. One known method for locating transmitters is time difference 3S of arri~al, tdoa, which has been used for many years, at least in such applications as LORAN and GPS. The w096/02006 PCT~S95/08526 ~17~325 2 application of this process to cell phones includes measuring the time of arrival of the same signal at a multiplicity of base stations and comparing the times to determine the difference between times of arrival and S thereby the differences in trAn~ission time from the cell phone or transmitter. Because it operates on the ordinary signal transmitted, it does not re~uire any modifications of the transmitter. One such application of tdoa technology combined with direction finding and applied to cellular phQ~e~ is described in U.S. Patent No. 5,317,323. The system described therein uses GPS
timing signals at the receivers to det~rmine the time of signal reception. Direction f; n~i ng is used to help eliminate multipath and co-channel interference.
lS Unfortunately, the use of direction f; n~i ng reguires the use of a steerable antenna or array and is thus not compatible with existing omnidirectional antenna structures. The patent does show the importance of removing multipath interference. Thus, it is a goal of any such locating sy-qtem for cellular telepho~es to distinguish between a direct signal from a cell phone and multipath reflections of the same signal from buildings and other reflectors. Reflected signals, which take a longer and unknown path to the receiver, provide less accurate location information.
S~MNARY OF THE lNv~ ON
Accordingly, it is a object of the present invention to provide an apparatus and method for determ;n;ng the location of cellular telephones by using time difference of arrival measurements at a multiplicity of cell phone base stations.
It is a further object of the present invention to perform such location f;n~ing even when the signals W096/02006 PCT~39SM8~?~
~ 2171325 3 ~
received at one or more of the base stations are indistinguishable from noise and other co-channel interference.
It is still a further object of the present S invention to perform such location finding in the presence of high levels of multipath reflected signals.
It is yet a further object of the present invention to perform such location f;n~ing while taking advantage of the frequency separation of a plurality of cell phone channels to improve the accuracy of the location finding.
It is yet a further object of the present invention to perform such location fin~ing using antennas and antenna structures which are normally used for typical cell phone operation.
lS In one form, the present invention provides a method for det~rminin~ the time difference of arrival, tdoa, of signals from a cellular telephone, cell pho~e, or similar transmitter at a pair of base stations, comprisin~ the steps of: substantially simultaneously sampling transmission signals from the cell pho~e or similar transmitter at the pair of base stations in a plurality of ~hAnnelg having different frequencies; correlating the correspo~i ng signals sampled at the base stations for each channel to determi n~ a tdoa therebetween for each channel; averaging the tdoas det~rm;ned for each chAnnel;
subtracting the average tdoa from each of the determined channel tdoas to det~rmine a residual phase tdoa for each channel; determ; n; ng an overall phase tdoa between the pair of base stations based upon the residual phase tdoas for each channel; and A~; ng the average tdoa to the overall phase tdoa to determ;ne an overall tdoa between the pair of base stations.
In another form, the present invention provides a method for collecting transmission signals at a first w096/02006 PCT~s95/08526 ~ t 32~ 4' cell phone base station from a particular cell phone for the purpose of determ;n;ng location of the cell phone using time difference of arrival, tdoa, comprising the 3teps of: receiving signals transmitted over all channels S of a wideband, cell phone base station, operating spectrum including a known reverse control channel thereof; digitizing the received wi~Ph~nA spectrum signals; storing the digitized signalQ as digitized data in a delay memory; monitoring the known reverse control channel signal for a cell phone transmission including a predeterm;ne~ number of either a cell phone to be located or a phone number being called; identifying a predetermined number from the monitoring of the reverse control ~hAnnel; and storing data from the delay memory lS in response to the identifying of the predetermined number, including the portion of the received signal bearing identifying data of the predetermi~e~ number.
In yet another form, the present invention provides an apparatus for collecting transmission signals at a first cell phone ba~e station from a particular cell phone for the purpose of determ;n;ng location of the cell phone using time difference of arrival, tdoa, comprising:
means for receiving signals transmitted over all channels of a wiAPh~nA, cell phone base station, operating spectrum including a known reverse control ch~nnel thereof; mP~n-s for digitizing the received wiAPb~nA
spectrum signalQ; means for storing the digitized signals as digitized data in a delay ~..~.IOLy; me~n~ for monitoring the known reverse control rh~nnel signal for a cell phone transmission including a predeterm;neA number of either a cell phone to be located or a number being called; means for identifying a predetPrmineA number from the monitoring of the reverse control rh~nnel; and means for storing data from the delay memory in response to the ~1 7~32~
identifying of the predetermined number, including the portion of the received signal bearing the identifying data of the predetermined number.
S BRI~F DEg~RIPTION OF TH~ DRAWINGg The present invention is illustratively described in reference to the appended drawings in which:
Fig. 1 is a system block diagram of an apparatus constructed in accordance with one embodiment of the present invention;
Fig. 2 is a block diagram of one embo~;ment of a portion of Fig. 1;
Fig. 3 is a block diagram of a second embQdiment of a portion of Fig. 1;
lS Fig. 4 is a signal diagram of some of the results of the tdoa determ;n~tion process;
Fig. 5 is a diagram of additional results of the tdoa process; and Fig. 6 is another signal diagram of additional results of the tdoa process.
DE~Tr-~n DEscR~ N OF T~E DRAWINGg Fig. 1 shows one ~oA; ment of a cell rhon~ locating system 10 which would take advantage of existing cell phone base stations and antennas. System 10 generally includes a multiplicity of existing cellular base stations 12-14, to which the apparatus of the present invention is added. Ba~e stations 12-14 make use of existing transmit and receive antennas 16 which may be identical in form and function. Added to each base station is an additional GPS (Global Positioning System) antenna 18 which receives timing signals from the existing civilian GPS for use in the locating function.
Each of the base stations also includes further equipment w096/02006 PCT~S95/08S26 ~71~ 6 for performing the locating function as described below.
~ommlln;cation lines 19 are shown connecting at least two remote base stations 12,13 with a central base station 14. Central base station 14 includes e~uipment not S present in the remote base stations 12,13 in the form of tdoa CPU 20 for proces~ing signals received by all base stations 12-14. Tdoa CPU 20 may be located at any one of the base stations or may be located separately from such base stations. Such separate location would include the interconnection of all comm. lines 19 directly between the ba~e stations and tdoa CPU 20. A cell phone 21, located within the range of the base stations 12-14, may be located by the equipment of the present embo~;m~nt.
The base station 14, having tdoa CPU 20, is shown lS being accessed by a computer 22, such as a portable computer, using a teleromm~ln;cations link, such as a cellular telPphQ~e modem 24, for the purpose of re~uesting location information or the performance of a locating operation on a cell phone such as 21.
Fig. 2 shows a system block diagram for installation at one of the remote base stations 12-14 of Fig. 1. The system 30 shown generally includes a receiver section 32, a processing section 34 and a control and comm~ln;cation section 36. Receiver section 32 is coupled to the existing cell phone antenna 16 and the GPS antenna 18.
Signals from the existing cell phone antenna 16 are coupled through a Filter and Low Noise Amplifier 40 to a fre~uency down converter 42 and a receiver 44 for the reverse control channel of the cell phone system. Down converter 42 outputs signals from the full cell phone bandwidth being received. The analog output of receiver 44 is n~mi n~l ly fed through a TTL converter 45 for providing a digital format-~to the cont~ol section 36.
WO 96/02006 2 ~ 7 1 3 2 ~ PCr/U~95108526 Down converter 42 also receives demodulated GPS
timing signals or time tags at one pulse per second and one kilopulse per second from a GPS receiver 46 connected to GPS antenna 18. Down converter 42 includes a counter (not shown) which is clocked hy the kilopulse signal and reset by the pulse per second signal, thus producing a time tag for association with the received cell phone signals. Thus, the GPS time tags, which are substantially the same at all local base stations are used to measure the time of arrival of signals received hy antenna 16 on both the reverse control chAnn~l 3 and the reverse voice channels. The received cell phone signals from down converter 42 are synchronously fed through an analog-to-digital, A/D, converter 48 to the lS processing section 34 along with the time tag signals on line 49. Additionally, A/D converter 48 is triggered in response to the GPS signals so that the time tags correspond to the exact points in time that A/D converter 48 performs sampling.
Processing section 34 generally includes a delay memory 50, a filter/decimator 52, a FIFO buffer 54 and a time tag decoder 56. Memory 50 receives digitized data from the cell phone hAn~width being received along with corresponding time tag data. Buffer 54 and decoder 56 output data to a control CPU 60 in control ection 36 and receive an ~l~rm or trigger signal therefrom.
Filter/decimator 52 receives a tuning signal from CPU 60.
The purpose of delay ~ .o~y 50 is to store dat~ from the cell phone h~n~-ridth while control CPU 60 det~rmines if a portion of that data and its correspon~; ng time tag should be recorded. Because delay memory 50 temporarily stores all data in the cell phone spectrum, it must operate at a fairly-high speed such as 25MHz to cover a cell phone h~n~-~ridth of lOMHz. The purpose of W096/02006 PCT~S95/08526 ~ 8 filter/decimator 52 is to digitally tune to only a single channel, 30KHz, and filter out the r~mA;n~er of the cell phone spectrum being fed from delay memory 50. secause the data then recorded in FIFO buffer S4 only corresponds S to the single channel bandwidth, it can run at a much lower speed, i.e. lOOKHz, than delay memory 50.
Control section 36 includes the control CPU 60 along with a co~m~ln;cations CPU 62 which commlln;cate with each other and with the r~m~;n~e~ of the locator system 10 using an Ethernet connection 64 over comm. lines 19.
Comm. CPU 62 is also coupled to a cell phone receiver 66 of sorts which solely monitors the forward control and voice channels and records voice channel assignments sent from the base station to individual cell phones. In another implementation voice channel assignment data may be hard wired or reported via a direct computer link from the cell phone system thereby making receiver 66 unnecessary.
In operation, comms. CPU 62 receives data over the Ethernet 64 to monitor for a specific predetermined phone number to be located. Cell phone calls to specific telephones number may also be so designated and monitored. Comms. CPU 62 passes this information to control ~PU 60 which monitors the reverse control channel signal from receiver 44. Control CPU 60 also ~ends a tuning signal to filter/decimator 52 so that the data exiting therefrom is the contents of the reverse control channel. Whenever a cell phone places a call, it transmits both its own number and the number to be called over the reverse control rh~nnel. Likewise whenever a cell phone is called, its number is transmitted over the forward control rh~nnel and it responds over the rever~e control ~h~nn~l using its phone number.
~ w096,~2-A6 2 1 7 1 3 2 ~ PCT~Sg5/08526 Whenever control CPU 60 m~tches a cell phone transmission number with either a predetermined number to be located or a specific number being called, such as '911', control CPU 60 generates an alarm or trigger S signal for FIFO buffer 54 and time tag decoder 56. This trigger causes buffer 54 to begin collecting data and decoder 56 to identify the specific time tag corresponding to the beg; nn; ng of the data being so stored. The trigger signal is simultaneously sent to all of the imme~;ately ~urrounding base stations so that simultaneously received signals are collected.
Surrounding base stations will collect data even though the reverse control channel signals they received may not have been strong enough to enable identification of the lS predetermined cell phone number or number being called.
The delay in time between the receipt of a reverse control channel signal and the decoding of that signal by control CPU 60 to generate a trigger in the present embo~im~nt amounts to a~oximately 20 ms. of data or le~s. This corresponds to the amount of data stored in delay memory 50 which can therefore be captured in buffer 54 even after the number being monitored ha-~ been decoded therefrom. In response to a trigger, buffer 54 collects 82 millisecond~ of data which may then be passed as a block or packet to control CPU 62 along with the corresponding ~coAeA initial time tag from decoder 56.
Once a monitored phone or a called number is identified from the reverse control rh~nnel from receiver 44, the forward control channel is further monitored by comms. CPU 62 and receiver 66 to determine the voice ~hAnnel assignment from the base station. This assignment is passed to control CPU 60. After the 82 milliseconds of reverse control ch~n~el data is collected in buffer 54, control CPU 60 re-tunes filter/~c;m~tor 52 ~i W O 96/02006 PCTnUS95/08526 2~ 7 13~S
to the assigned reverse voice channel and re-triggers buffer 54 and time tag decoder 56 to collect another 82 milliseconds of data from the assigned reverse voice channel. This data is likewise passed with its initial S time tag to control CPU 60 and the locator system for processing.
During and after data collection from the reverse voice channel, receiver 66 and comms. CPU 62 also monitor the forward voice channel to detect further voice channel assignments. When such assignments are detected, control CPU 60, as well as surrounding base stations, are notified to change data collection ch~nnels. This subseguent reverse voice ch~nnel switching both insures that sufficient data is collected for each collection period and also provides additional data for improving the accuracy of the location function performed by tdoa CPU 20.
This process of recording reverse control and voice ~h~nnel data with its corresponding time tags is performed simultaneously at each of the base stations receiving the cell phone transmission. This may be done in response to identification of the reverse control channel signal at each base station or it may be done in response to trigger signals generated from one or more of the base stations and passed to surro~Aing base stations via comms. CPU 62 and the Ethernet connection 64. The latter triggering avoids the necessity that each of the base stations receive a signal of sufficient -~trength from which to identify the predeterm;nP~ number or the called number. The 82 milliseconds of recorded data at each base station provides sufficient identity between the three recorded sets of data to determ;ne comparable times of arrival. Thereafter the recorded data is passed to tdoa CPU 20 where calculation~ are made to det~rm;ne W O 96/02006 ~ 1 7 1 ~ ~ ~ PC~rrUS95/08526 , relative transmission time to the base stations 12-14 and to apply that data to mapping software. These tdoa calculations are described in greater detail below.
Fig. 3 shows an alternate system ~m~o~;ment 70 of S the base station system 30 of Fig. 2. Section~ and components in Fig. 3 bearing the same reference numbers as those in Fig. 2 are identical in form and function.
Generally included are a receiver section 72, processing section 74 and control and commllnication section 76.
Receiver section 72 differs from Fig. 2 in that it includes a dual down converter 78 and a pair of A/D
converters 80,81. The purpose for this is to cover the entire 25 MHz cell phone spectrum with available A/D
converters 80,81 which only operate at 30 MHz. This is lS accompl;she~ by splitting the cell phone spectrum into a pair of 12.5 MHz. base band signals with the dual down converter 78 and then feeding each base band signal through a separate 30 MHz. A/D 80,81 to provide adequate sampling. To control the A/D converters 80,81 and time tag generation, a 10 MHz. signal from the GPS receiver 46 is converted to a 30 MHz clock signal by the down converter and fed to converters 80,81 and a time tag generator 82 in processor section 74.
Processor section 74 further includes a delay memory 84, a filter/decimator 54, FIFO buffer 84 and an FM
demodulator 86. Control and c~ nication section 76 includes a single CPU 88 to perform the functions of both CPUs of Fig. 2. The type of signals passed between processor section 74 and control and comm. section 76 are the same as Fig. 2.
In operation, all of the channels in the wi~
cell phone spectrum are received, down converted and digitized in receiver section 72 and fed-to processor section 74 as a pair of base band signals 90,91 along W0 96/02006 s ~~ PCT/US95/08526 ~7~325 12 with clock and time tag signals synchronized to the digitization process. Processor section 74 receives one of the digitized data signals 90,91, under the control of CPU 88 dep~n~; n~ upon the spectrum location of the reverse control rhAnnel being monitored. The received signal 90,91 is fed directly into delay memory 82 and into the filter/decimator 54 through a shunt 92 around delay memory 82. Filter/decimator 54 is initially ~-et to separate the reverse control channel and feeds that data to FM ~Pmo~l~lator 86, which separates the data contained therein from the carrier signal so that the data can be monitored by CPU 88. This data is pa~sed through a small portion of buffer 84 and fed to CPU 88 for monitoring.
When CPU 88 detects a response from a predetermined lS cell phone, or a predet~rm;neA called number, over the demodulated reverse control channel, it generates a trigger which ~witches the input of filter/decimator 54 from shunt 92 to the output of delay memory 82. Delay memory 82 holds sufficient data so that the ~redet~rm;ne~
number may be detected by CPU 88 while the same data is still present in delay memory 82. The trigger signal also causes a time tag signal to be sent to CPU 88 to mark the beg; nn; ng of the data being collected and causes the data output of filter/decimator to be stored in FIFO
buffer 84. Once an amount of data (nnm;n~l ly 320 milliseconds) has been collected in buffer 84, it is passed to CPU 88. CPU 88 then re-tunes the filter/decimator to the reverse voice ch~nnel assigned to the cell phone and detected from receiver 66 and also triggers another time tag. This causes data to be collected in buffer 84 from the reverse voice ch~nn~1.
Similar data collections are also made from subse~uent reverse voice ch~nn~l assignments. Once data has been collected and sent to CPU 88, the data is sent with its WO 9G/02~06 21 713 2 ~ PCT/US95/~85~6 respective time tags to tdoa CPU 20 for tdoa calculations.
The method and apparatuses which function in accordance with the present invention are robust in that S only one of the base stations monitoring the reverse control channel needs to clearly receive a signal identifying either a predetermined cell phone or a called phone. Because it is possible to simply trigger all surrounding base stations to record and transmit data based upon one or more identified control and voice channels, even signals received by several of the base stations which are too weak for identification can still be used for tdoa measurements. The remote triggering is facilitated by the wi~ph~n~ nature of the data which is lS received and stored in the delay memory of each base station.
The purpose of analyzing the signal data from each of th~ ba-~e stations is to calculate the tdoa between at least two pairs out of at least three base stations.
This tdoa data is then converted to relative distance data and the location of the transmitter can be calculated from the known locations of the base stations and coordinated with map software. For purposes of identification and notation, base stations are referred to as a,b,c and cell phone transmis~ion channels located at different freguencies in the cell phone spectrum are referred to as x,y,z.
The signal data, S(t) for tdoa calculations comes in packets of diqitized signal~. Each packet, S, corresponds to one tr~ncm;ssion channel x,y,z received at one base station, a,b,c as in Sax,Say~saz~sbx.... Each packet also includes a time tag, nomi n~l ly for the first sample in the packet, and the sampling frequency or time between samples is known. Two or more ch~nnel w096/02006 ~ s;~ PCT~s95/08526 2,~ 7 ~2~
transmissions are sampled at each base station. Each channel transmission is qampled at three base stations or more.
The first step in the tdoa determ;nation process is S the transformation of each packet of data into the frequency ~m~in by performing an FFT to produce S(f) as in Sax,Say,Saz~Sbx~Sby~sbz----The overall tdoa between any two base stations, rab is determined by first determ;n;ng the tdoa function in each separate transmission channel, Rabx,Raby,Rabz- This is done in the frequency ~o~; n by multiplying the signal data received at a base station in a transmission channel, SaX, by the complex conjugate of the same channel data received at another base station, SbX*~
RabX = Sax X SbX*
The same correlation is performed for each transmission channel, x,y,z, between the two base stations a,b.
These calculated tdoa functions are converted back into the time ~om~; n by an inverse FFT and show an amplitude peak, as seen in Fig. 4, at the tdoa of the two correlated signals. The precise peak is usually not aligned with one of the points produced by the inverse FFT, so a quadratic interpolation is performed using the adjacent points to make a more accurate det~rm;n~tion of the peak amplitude and the exact time delay, or tdoa, thereof for each transmission rh~nnel.
The tdoa signals plotted in Fig. 4 vary between tr~n~mission channel~ for each pair of base stations because of phase differences at the different frequencies. However, the tdoa amplitudes in the time ~om~; n are substantially equal in the absence of substantial multipath signal reception. Thus, variation w096l0~006 ~ 2 1 7 1 3 2 ~ PCT~S95/08~6 of the tdoa amplitudes between transmission channels is an indication of multipath problems.
The interpolated time delay or tdoa values for all transmission channels measured between two base stations S are averaged to determine Avg.rab.
The average tdoa is subtracted from each of the correlated tdoa functions in the frequency ~om~i n ky using a multiplication factor to determine a residual phase tdoa, Ph .RabX~ due to phase in each ch~nnel.
Ph.RabX = RabX x ej2Pift~ where t = Avg.rab, and f is the center frequency of the respective channel, x.
The resulting phase vectors in each of the frequency bins of the residual tdoa function, Ph.Rabx~ Ph-Raby,, are summed to determine the residual phase tdoa, Ph.Rab, for each transmission channel, x,y,z.
The amplitude of the residual phase tdoas from all of the transmission channels between two base stations are plotted, as shown in Fig. 5, against their respective transmission channel frequencies and the angle of the plot determines the overall residual tdoa, Ph.rab, between two base stations. This residual phase tdoa is added to the average tdoa, Avg.rab, for the respective pair of base stations to determine the overall tdoa, r between those stations.
If the plot of phase tdoa vs. frequency of Fig. 5 is not linear or if the peak values of the tdoa functions in the time ~nmA; n of Fig. 4 are not egual between a pair of base stations, multipath, or multiple signal reception is indicated. In such case, the residual phase tdoa values for each frequency chAnnel~ as shown in Fig. 5, are combined with their respective interpolated amplitude values from RabX~... of Fig. 4-and are-noted--in the frequency ~omA;n. All nonsample frequencies are set to a W096/02006 ^ PCT~Sg5/08526 ~7 ~325 ~
zero amplitude and zero phase. The resulting frequency ~m~; n data is converted back into the time ~om~ i n by an inverse FFT and the resulting time ~nm~; n data will show the receipt of multiple signals as shown in Fig. 7. The S earlier signal 94 is used as the overall residual phase delay, Ph.rab, even thou~h the later signal 96 may be stronger. With a m; n;mllm of three transmission channels unevenly spaced within the cell phone band, the earlier arrival may be readily ascertained. Distinguishing the various multipath signals received is also enhanced by zero padding in the freguency ~m~;n before taking the inverse FFT.
The embodiments described above are intended to be taken in an illustrative and not a limiting ~ense.
lS Various modifications and changes may be made to the above embo~;m~nts by persons ~killed in the art without departing from the scope of the present invention as defined in the appended claims. For example, the present invention may be applied for locating similar transmitters other than cellular phones and transmitters using different transmission formats. ~hese include personal commlln;cation system transmitters, or PCS, and code division packet data, or CDPD.
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APPARAT~S AND h~ ~iOv FOR ~OCATING r~rr~T~ TEL~r~o~.c AND STMTr.~R TRANSh~ SKS
S Bach~Ou~d of the Invention Field of the Invention The present invention generally relates to the function of locating cellular telephones and similar mobile transmitters, and particularly, to a method and apparatus which use time-difference-of-arrival techniques and which take advantage of the multiple channels used by cellular telephones and similar transmitters for this purpose.
lS Statement of the Prior Art The proliferation of cellular telephones, or cell phones, and their technology and usage, has revealed various applications for the ability to locate cell phones. These applications include ~911~ calls, tourist and travel information, tracking of unauthorized cell phone usage and illegal activities, and the locating of commercial and government vehicles, to name a few. The basic cell phone systems can only determine the nearest cell phone base station, which locates the cell phone to 2S within 3 to 10 miles.
A goal of any such system for use with cellular phones is the use of as much existing e~uipment as possible or at least compatibility with existing e~uipment. Examples of this are use with unmodified cellular phQ~e and compatibility with existing cellular base stations.
A great deal of technology already exists concerning the tracking or locating of radio transmitters. One known method for locating transmitters is time difference 3S of arri~al, tdoa, which has been used for many years, at least in such applications as LORAN and GPS. The w096/02006 PCT~S95/08526 ~17~325 2 application of this process to cell phones includes measuring the time of arrival of the same signal at a multiplicity of base stations and comparing the times to determine the difference between times of arrival and S thereby the differences in trAn~ission time from the cell phone or transmitter. Because it operates on the ordinary signal transmitted, it does not re~uire any modifications of the transmitter. One such application of tdoa technology combined with direction finding and applied to cellular phQ~e~ is described in U.S. Patent No. 5,317,323. The system described therein uses GPS
timing signals at the receivers to det~rmine the time of signal reception. Direction f; n~i ng is used to help eliminate multipath and co-channel interference.
lS Unfortunately, the use of direction f; n~i ng reguires the use of a steerable antenna or array and is thus not compatible with existing omnidirectional antenna structures. The patent does show the importance of removing multipath interference. Thus, it is a goal of any such locating sy-qtem for cellular telepho~es to distinguish between a direct signal from a cell phone and multipath reflections of the same signal from buildings and other reflectors. Reflected signals, which take a longer and unknown path to the receiver, provide less accurate location information.
S~MNARY OF THE lNv~ ON
Accordingly, it is a object of the present invention to provide an apparatus and method for determ;n;ng the location of cellular telephones by using time difference of arrival measurements at a multiplicity of cell phone base stations.
It is a further object of the present invention to perform such location f;n~ing even when the signals W096/02006 PCT~39SM8~?~
~ 2171325 3 ~
received at one or more of the base stations are indistinguishable from noise and other co-channel interference.
It is still a further object of the present S invention to perform such location finding in the presence of high levels of multipath reflected signals.
It is yet a further object of the present invention to perform such location f;n~ing while taking advantage of the frequency separation of a plurality of cell phone channels to improve the accuracy of the location finding.
It is yet a further object of the present invention to perform such location fin~ing using antennas and antenna structures which are normally used for typical cell phone operation.
lS In one form, the present invention provides a method for det~rminin~ the time difference of arrival, tdoa, of signals from a cellular telephone, cell pho~e, or similar transmitter at a pair of base stations, comprisin~ the steps of: substantially simultaneously sampling transmission signals from the cell pho~e or similar transmitter at the pair of base stations in a plurality of ~hAnnelg having different frequencies; correlating the correspo~i ng signals sampled at the base stations for each channel to determi n~ a tdoa therebetween for each channel; averaging the tdoas det~rm;ned for each chAnnel;
subtracting the average tdoa from each of the determined channel tdoas to det~rmine a residual phase tdoa for each channel; determ; n; ng an overall phase tdoa between the pair of base stations based upon the residual phase tdoas for each channel; and A~; ng the average tdoa to the overall phase tdoa to determ;ne an overall tdoa between the pair of base stations.
In another form, the present invention provides a method for collecting transmission signals at a first w096/02006 PCT~s95/08526 ~ t 32~ 4' cell phone base station from a particular cell phone for the purpose of determ;n;ng location of the cell phone using time difference of arrival, tdoa, comprising the 3teps of: receiving signals transmitted over all channels S of a wideband, cell phone base station, operating spectrum including a known reverse control channel thereof; digitizing the received wi~Ph~nA spectrum signals; storing the digitized signalQ as digitized data in a delay memory; monitoring the known reverse control channel signal for a cell phone transmission including a predeterm;ne~ number of either a cell phone to be located or a phone number being called; identifying a predetermined number from the monitoring of the reverse control ~hAnnel; and storing data from the delay memory lS in response to the identifying of the predetermined number, including the portion of the received signal bearing identifying data of the predetermi~e~ number.
In yet another form, the present invention provides an apparatus for collecting transmission signals at a first cell phone ba~e station from a particular cell phone for the purpose of determ;n;ng location of the cell phone using time difference of arrival, tdoa, comprising:
means for receiving signals transmitted over all channels of a wiAPh~nA, cell phone base station, operating spectrum including a known reverse control ch~nnel thereof; mP~n-s for digitizing the received wiAPb~nA
spectrum signalQ; means for storing the digitized signals as digitized data in a delay ~..~.IOLy; me~n~ for monitoring the known reverse control rh~nnel signal for a cell phone transmission including a predeterm;neA number of either a cell phone to be located or a number being called; means for identifying a predetPrmineA number from the monitoring of the reverse control rh~nnel; and means for storing data from the delay memory in response to the ~1 7~32~
identifying of the predetermined number, including the portion of the received signal bearing the identifying data of the predetermined number.
S BRI~F DEg~RIPTION OF TH~ DRAWINGg The present invention is illustratively described in reference to the appended drawings in which:
Fig. 1 is a system block diagram of an apparatus constructed in accordance with one embodiment of the present invention;
Fig. 2 is a block diagram of one embo~;ment of a portion of Fig. 1;
Fig. 3 is a block diagram of a second embQdiment of a portion of Fig. 1;
lS Fig. 4 is a signal diagram of some of the results of the tdoa determ;n~tion process;
Fig. 5 is a diagram of additional results of the tdoa process; and Fig. 6 is another signal diagram of additional results of the tdoa process.
DE~Tr-~n DEscR~ N OF T~E DRAWINGg Fig. 1 shows one ~oA; ment of a cell rhon~ locating system 10 which would take advantage of existing cell phone base stations and antennas. System 10 generally includes a multiplicity of existing cellular base stations 12-14, to which the apparatus of the present invention is added. Ba~e stations 12-14 make use of existing transmit and receive antennas 16 which may be identical in form and function. Added to each base station is an additional GPS (Global Positioning System) antenna 18 which receives timing signals from the existing civilian GPS for use in the locating function.
Each of the base stations also includes further equipment w096/02006 PCT~S95/08S26 ~71~ 6 for performing the locating function as described below.
~ommlln;cation lines 19 are shown connecting at least two remote base stations 12,13 with a central base station 14. Central base station 14 includes e~uipment not S present in the remote base stations 12,13 in the form of tdoa CPU 20 for proces~ing signals received by all base stations 12-14. Tdoa CPU 20 may be located at any one of the base stations or may be located separately from such base stations. Such separate location would include the interconnection of all comm. lines 19 directly between the ba~e stations and tdoa CPU 20. A cell phone 21, located within the range of the base stations 12-14, may be located by the equipment of the present embo~;m~nt.
The base station 14, having tdoa CPU 20, is shown lS being accessed by a computer 22, such as a portable computer, using a teleromm~ln;cations link, such as a cellular telPphQ~e modem 24, for the purpose of re~uesting location information or the performance of a locating operation on a cell phone such as 21.
Fig. 2 shows a system block diagram for installation at one of the remote base stations 12-14 of Fig. 1. The system 30 shown generally includes a receiver section 32, a processing section 34 and a control and comm~ln;cation section 36. Receiver section 32 is coupled to the existing cell phone antenna 16 and the GPS antenna 18.
Signals from the existing cell phone antenna 16 are coupled through a Filter and Low Noise Amplifier 40 to a fre~uency down converter 42 and a receiver 44 for the reverse control channel of the cell phone system. Down converter 42 outputs signals from the full cell phone bandwidth being received. The analog output of receiver 44 is n~mi n~l ly fed through a TTL converter 45 for providing a digital format-~to the cont~ol section 36.
WO 96/02006 2 ~ 7 1 3 2 ~ PCr/U~95108526 Down converter 42 also receives demodulated GPS
timing signals or time tags at one pulse per second and one kilopulse per second from a GPS receiver 46 connected to GPS antenna 18. Down converter 42 includes a counter (not shown) which is clocked hy the kilopulse signal and reset by the pulse per second signal, thus producing a time tag for association with the received cell phone signals. Thus, the GPS time tags, which are substantially the same at all local base stations are used to measure the time of arrival of signals received hy antenna 16 on both the reverse control chAnn~l 3 and the reverse voice channels. The received cell phone signals from down converter 42 are synchronously fed through an analog-to-digital, A/D, converter 48 to the lS processing section 34 along with the time tag signals on line 49. Additionally, A/D converter 48 is triggered in response to the GPS signals so that the time tags correspond to the exact points in time that A/D converter 48 performs sampling.
Processing section 34 generally includes a delay memory 50, a filter/decimator 52, a FIFO buffer 54 and a time tag decoder 56. Memory 50 receives digitized data from the cell phone hAn~width being received along with corresponding time tag data. Buffer 54 and decoder 56 output data to a control CPU 60 in control ection 36 and receive an ~l~rm or trigger signal therefrom.
Filter/decimator 52 receives a tuning signal from CPU 60.
The purpose of delay ~ .o~y 50 is to store dat~ from the cell phone h~n~-ridth while control CPU 60 det~rmines if a portion of that data and its correspon~; ng time tag should be recorded. Because delay memory 50 temporarily stores all data in the cell phone spectrum, it must operate at a fairly-high speed such as 25MHz to cover a cell phone h~n~-~ridth of lOMHz. The purpose of W096/02006 PCT~S95/08526 ~ 8 filter/decimator 52 is to digitally tune to only a single channel, 30KHz, and filter out the r~mA;n~er of the cell phone spectrum being fed from delay memory 50. secause the data then recorded in FIFO buffer S4 only corresponds S to the single channel bandwidth, it can run at a much lower speed, i.e. lOOKHz, than delay memory 50.
Control section 36 includes the control CPU 60 along with a co~m~ln;cations CPU 62 which commlln;cate with each other and with the r~m~;n~e~ of the locator system 10 using an Ethernet connection 64 over comm. lines 19.
Comm. CPU 62 is also coupled to a cell phone receiver 66 of sorts which solely monitors the forward control and voice channels and records voice channel assignments sent from the base station to individual cell phones. In another implementation voice channel assignment data may be hard wired or reported via a direct computer link from the cell phone system thereby making receiver 66 unnecessary.
In operation, comms. CPU 62 receives data over the Ethernet 64 to monitor for a specific predetermined phone number to be located. Cell phone calls to specific telephones number may also be so designated and monitored. Comms. CPU 62 passes this information to control ~PU 60 which monitors the reverse control channel signal from receiver 44. Control CPU 60 also ~ends a tuning signal to filter/decimator 52 so that the data exiting therefrom is the contents of the reverse control channel. Whenever a cell phone places a call, it transmits both its own number and the number to be called over the reverse control rh~nnel. Likewise whenever a cell phone is called, its number is transmitted over the forward control rh~nnel and it responds over the rever~e control ~h~nn~l using its phone number.
~ w096,~2-A6 2 1 7 1 3 2 ~ PCT~Sg5/08526 Whenever control CPU 60 m~tches a cell phone transmission number with either a predetermined number to be located or a specific number being called, such as '911', control CPU 60 generates an alarm or trigger S signal for FIFO buffer 54 and time tag decoder 56. This trigger causes buffer 54 to begin collecting data and decoder 56 to identify the specific time tag corresponding to the beg; nn; ng of the data being so stored. The trigger signal is simultaneously sent to all of the imme~;ately ~urrounding base stations so that simultaneously received signals are collected.
Surrounding base stations will collect data even though the reverse control channel signals they received may not have been strong enough to enable identification of the lS predetermined cell phone number or number being called.
The delay in time between the receipt of a reverse control channel signal and the decoding of that signal by control CPU 60 to generate a trigger in the present embo~im~nt amounts to a~oximately 20 ms. of data or le~s. This corresponds to the amount of data stored in delay memory 50 which can therefore be captured in buffer 54 even after the number being monitored ha-~ been decoded therefrom. In response to a trigger, buffer 54 collects 82 millisecond~ of data which may then be passed as a block or packet to control CPU 62 along with the corresponding ~coAeA initial time tag from decoder 56.
Once a monitored phone or a called number is identified from the reverse control rh~nnel from receiver 44, the forward control channel is further monitored by comms. CPU 62 and receiver 66 to determine the voice ~hAnnel assignment from the base station. This assignment is passed to control CPU 60. After the 82 milliseconds of reverse control ch~n~el data is collected in buffer 54, control CPU 60 re-tunes filter/~c;m~tor 52 ~i W O 96/02006 PCTnUS95/08526 2~ 7 13~S
to the assigned reverse voice channel and re-triggers buffer 54 and time tag decoder 56 to collect another 82 milliseconds of data from the assigned reverse voice channel. This data is likewise passed with its initial S time tag to control CPU 60 and the locator system for processing.
During and after data collection from the reverse voice channel, receiver 66 and comms. CPU 62 also monitor the forward voice channel to detect further voice channel assignments. When such assignments are detected, control CPU 60, as well as surrounding base stations, are notified to change data collection ch~nnels. This subseguent reverse voice ch~nnel switching both insures that sufficient data is collected for each collection period and also provides additional data for improving the accuracy of the location function performed by tdoa CPU 20.
This process of recording reverse control and voice ~h~nnel data with its corresponding time tags is performed simultaneously at each of the base stations receiving the cell phone transmission. This may be done in response to identification of the reverse control channel signal at each base station or it may be done in response to trigger signals generated from one or more of the base stations and passed to surro~Aing base stations via comms. CPU 62 and the Ethernet connection 64. The latter triggering avoids the necessity that each of the base stations receive a signal of sufficient -~trength from which to identify the predeterm;nP~ number or the called number. The 82 milliseconds of recorded data at each base station provides sufficient identity between the three recorded sets of data to determ;ne comparable times of arrival. Thereafter the recorded data is passed to tdoa CPU 20 where calculation~ are made to det~rm;ne W O 96/02006 ~ 1 7 1 ~ ~ ~ PC~rrUS95/08526 , relative transmission time to the base stations 12-14 and to apply that data to mapping software. These tdoa calculations are described in greater detail below.
Fig. 3 shows an alternate system ~m~o~;ment 70 of S the base station system 30 of Fig. 2. Section~ and components in Fig. 3 bearing the same reference numbers as those in Fig. 2 are identical in form and function.
Generally included are a receiver section 72, processing section 74 and control and commllnication section 76.
Receiver section 72 differs from Fig. 2 in that it includes a dual down converter 78 and a pair of A/D
converters 80,81. The purpose for this is to cover the entire 25 MHz cell phone spectrum with available A/D
converters 80,81 which only operate at 30 MHz. This is lS accompl;she~ by splitting the cell phone spectrum into a pair of 12.5 MHz. base band signals with the dual down converter 78 and then feeding each base band signal through a separate 30 MHz. A/D 80,81 to provide adequate sampling. To control the A/D converters 80,81 and time tag generation, a 10 MHz. signal from the GPS receiver 46 is converted to a 30 MHz clock signal by the down converter and fed to converters 80,81 and a time tag generator 82 in processor section 74.
Processor section 74 further includes a delay memory 84, a filter/decimator 54, FIFO buffer 84 and an FM
demodulator 86. Control and c~ nication section 76 includes a single CPU 88 to perform the functions of both CPUs of Fig. 2. The type of signals passed between processor section 74 and control and comm. section 76 are the same as Fig. 2.
In operation, all of the channels in the wi~
cell phone spectrum are received, down converted and digitized in receiver section 72 and fed-to processor section 74 as a pair of base band signals 90,91 along W0 96/02006 s ~~ PCT/US95/08526 ~7~325 12 with clock and time tag signals synchronized to the digitization process. Processor section 74 receives one of the digitized data signals 90,91, under the control of CPU 88 dep~n~; n~ upon the spectrum location of the reverse control rhAnnel being monitored. The received signal 90,91 is fed directly into delay memory 82 and into the filter/decimator 54 through a shunt 92 around delay memory 82. Filter/decimator 54 is initially ~-et to separate the reverse control channel and feeds that data to FM ~Pmo~l~lator 86, which separates the data contained therein from the carrier signal so that the data can be monitored by CPU 88. This data is pa~sed through a small portion of buffer 84 and fed to CPU 88 for monitoring.
When CPU 88 detects a response from a predetermined lS cell phone, or a predet~rm;neA called number, over the demodulated reverse control channel, it generates a trigger which ~witches the input of filter/decimator 54 from shunt 92 to the output of delay memory 82. Delay memory 82 holds sufficient data so that the ~redet~rm;ne~
number may be detected by CPU 88 while the same data is still present in delay memory 82. The trigger signal also causes a time tag signal to be sent to CPU 88 to mark the beg; nn; ng of the data being collected and causes the data output of filter/decimator to be stored in FIFO
buffer 84. Once an amount of data (nnm;n~l ly 320 milliseconds) has been collected in buffer 84, it is passed to CPU 88. CPU 88 then re-tunes the filter/decimator to the reverse voice ch~nnel assigned to the cell phone and detected from receiver 66 and also triggers another time tag. This causes data to be collected in buffer 84 from the reverse voice ch~nn~1.
Similar data collections are also made from subse~uent reverse voice ch~nn~l assignments. Once data has been collected and sent to CPU 88, the data is sent with its WO 9G/02~06 21 713 2 ~ PCT/US95/~85~6 respective time tags to tdoa CPU 20 for tdoa calculations.
The method and apparatuses which function in accordance with the present invention are robust in that S only one of the base stations monitoring the reverse control channel needs to clearly receive a signal identifying either a predetermined cell phone or a called phone. Because it is possible to simply trigger all surrounding base stations to record and transmit data based upon one or more identified control and voice channels, even signals received by several of the base stations which are too weak for identification can still be used for tdoa measurements. The remote triggering is facilitated by the wi~ph~n~ nature of the data which is lS received and stored in the delay memory of each base station.
The purpose of analyzing the signal data from each of th~ ba-~e stations is to calculate the tdoa between at least two pairs out of at least three base stations.
This tdoa data is then converted to relative distance data and the location of the transmitter can be calculated from the known locations of the base stations and coordinated with map software. For purposes of identification and notation, base stations are referred to as a,b,c and cell phone transmis~ion channels located at different freguencies in the cell phone spectrum are referred to as x,y,z.
The signal data, S(t) for tdoa calculations comes in packets of diqitized signal~. Each packet, S, corresponds to one tr~ncm;ssion channel x,y,z received at one base station, a,b,c as in Sax,Say~saz~sbx.... Each packet also includes a time tag, nomi n~l ly for the first sample in the packet, and the sampling frequency or time between samples is known. Two or more ch~nnel w096/02006 ~ s;~ PCT~s95/08526 2,~ 7 ~2~
transmissions are sampled at each base station. Each channel transmission is qampled at three base stations or more.
The first step in the tdoa determ;nation process is S the transformation of each packet of data into the frequency ~m~in by performing an FFT to produce S(f) as in Sax,Say,Saz~Sbx~Sby~sbz----The overall tdoa between any two base stations, rab is determined by first determ;n;ng the tdoa function in each separate transmission channel, Rabx,Raby,Rabz- This is done in the frequency ~o~; n by multiplying the signal data received at a base station in a transmission channel, SaX, by the complex conjugate of the same channel data received at another base station, SbX*~
RabX = Sax X SbX*
The same correlation is performed for each transmission channel, x,y,z, between the two base stations a,b.
These calculated tdoa functions are converted back into the time ~om~; n by an inverse FFT and show an amplitude peak, as seen in Fig. 4, at the tdoa of the two correlated signals. The precise peak is usually not aligned with one of the points produced by the inverse FFT, so a quadratic interpolation is performed using the adjacent points to make a more accurate det~rm;n~tion of the peak amplitude and the exact time delay, or tdoa, thereof for each transmission rh~nnel.
The tdoa signals plotted in Fig. 4 vary between tr~n~mission channel~ for each pair of base stations because of phase differences at the different frequencies. However, the tdoa amplitudes in the time ~om~; n are substantially equal in the absence of substantial multipath signal reception. Thus, variation w096l0~006 ~ 2 1 7 1 3 2 ~ PCT~S95/08~6 of the tdoa amplitudes between transmission channels is an indication of multipath problems.
The interpolated time delay or tdoa values for all transmission channels measured between two base stations S are averaged to determine Avg.rab.
The average tdoa is subtracted from each of the correlated tdoa functions in the frequency ~om~i n ky using a multiplication factor to determine a residual phase tdoa, Ph .RabX~ due to phase in each ch~nnel.
Ph.RabX = RabX x ej2Pift~ where t = Avg.rab, and f is the center frequency of the respective channel, x.
The resulting phase vectors in each of the frequency bins of the residual tdoa function, Ph.Rabx~ Ph-Raby,, are summed to determine the residual phase tdoa, Ph.Rab, for each transmission channel, x,y,z.
The amplitude of the residual phase tdoas from all of the transmission channels between two base stations are plotted, as shown in Fig. 5, against their respective transmission channel frequencies and the angle of the plot determines the overall residual tdoa, Ph.rab, between two base stations. This residual phase tdoa is added to the average tdoa, Avg.rab, for the respective pair of base stations to determine the overall tdoa, r between those stations.
If the plot of phase tdoa vs. frequency of Fig. 5 is not linear or if the peak values of the tdoa functions in the time ~nmA; n of Fig. 4 are not egual between a pair of base stations, multipath, or multiple signal reception is indicated. In such case, the residual phase tdoa values for each frequency chAnnel~ as shown in Fig. 5, are combined with their respective interpolated amplitude values from RabX~... of Fig. 4-and are-noted--in the frequency ~omA;n. All nonsample frequencies are set to a W096/02006 ^ PCT~Sg5/08526 ~7 ~325 ~
zero amplitude and zero phase. The resulting frequency ~m~; n data is converted back into the time ~om~ i n by an inverse FFT and the resulting time ~nm~; n data will show the receipt of multiple signals as shown in Fig. 7. The S earlier signal 94 is used as the overall residual phase delay, Ph.rab, even thou~h the later signal 96 may be stronger. With a m; n;mllm of three transmission channels unevenly spaced within the cell phone band, the earlier arrival may be readily ascertained. Distinguishing the various multipath signals received is also enhanced by zero padding in the freguency ~m~;n before taking the inverse FFT.
The embodiments described above are intended to be taken in an illustrative and not a limiting ~ense.
lS Various modifications and changes may be made to the above embo~;m~nts by persons ~killed in the art without departing from the scope of the present invention as defined in the appended claims. For example, the present invention may be applied for locating similar transmitters other than cellular phones and transmitters using different transmission formats. ~hese include personal commlln;cation system transmitters, or PCS, and code division packet data, or CDPD.
Claims (17)
1. A method for determining the time difference of arrival, tdoa, of signals from a cellular telephone, cell phone, or similar transmitters at a pair of base stations, comprising the steps of:
substantially simultaneously sampling transmission signals from the cell phone or similar transmitter at the pair of base stations in a plurality of channels having different frequencies;
correlating the corresponding signals sampled at the base stations for each channel to determine a tdoa therebetween for each channel;
averaging the tdoas determined for each channel;
subtracting the average tdoa from each of the determined channel tdoas to determine a residual phase tdoa for each channel;
determining an overall phase tdoa between the pair of base stations based upon the residual phase tdoas for each channel; and adding the average tdoa to the overall phase tdoa to determine an overall tdoa between the pair of base stations.
substantially simultaneously sampling transmission signals from the cell phone or similar transmitter at the pair of base stations in a plurality of channels having different frequencies;
correlating the corresponding signals sampled at the base stations for each channel to determine a tdoa therebetween for each channel;
averaging the tdoas determined for each channel;
subtracting the average tdoa from each of the determined channel tdoas to determine a residual phase tdoa for each channel;
determining an overall phase tdoa between the pair of base stations based upon the residual phase tdoas for each channel; and adding the average tdoa to the overall phase tdoa to determine an overall tdoa between the pair of base stations.
2. The method of claim 1, wherein the step of correlating produces a tdoa function for each channel and further wherein the step of subtracting the average tdoa from each of the channel tdoas includes multiplication of the correlated tdoa functions for each channel by a factor in the frequency domain and vectorally summing the results thereof to produce a residual phase measurement for each channel.
3. The method of claim 2, wherein the step of determining an overall phase tdoa includes deriving an overall relationship between the residual phase measurements of all channels and their respective channel frequencies.
4. The method of claim 3, wherein the step of deriving an overall relationship includes plotting the residual phase measurements of all channels in the frequency domain against their respective channel frequencies, determining the angle of the plotted phase measurements with respect to the frequency axis, and determining the overall phase tdoa from the determined angle.
5. The method of claim 4, further comprising evaluating the presence of multipath signals from amplitudes of the correlated tdoa functions or from the derived overall relationship between the residual phase measurements of all channels and their respective frequencies.
6. The method of claim 2, wherein the step of determining an overall phase tdoa, comprises the steps of:
creating a frequency domain function from the residual phase measurements for each channel frequency and respective amplitudes of the correlated tdoa relationships for each channel;
inverse Fourier transforming the created frequency domain function into a time domain function; and choosing the overall phase tdoa from a peak occurrence in the transformed time domain function.
creating a frequency domain function from the residual phase measurements for each channel frequency and respective amplitudes of the correlated tdoa relationships for each channel;
inverse Fourier transforming the created frequency domain function into a time domain function; and choosing the overall phase tdoa from a peak occurrence in the transformed time domain function.
7. The method of claim 6, wherein the step of inverse Fourier transforming results in a time domain function having multiple peaks occurring at sequential points in time representing multiple signal reception, and further wherein the step of choosing the overall phase tdoa includes using a point in time of an earlier occurring peak as the overall phase tdoa.
8. The method of claim 6, wherein the step of creating a frequency domain signal includes setting amplitude and phase values of frequencies outside of the sampled plurality of channels equal to zero.
9. A method for collecting transmission signals at a first base station from a particular cell phone for the purpose of determining location of the cell phone using time difference of arrival, tdoa, comprising the steps of:
receiving signals transmitted over all channels of a wideband, cell phone base station operating spectrum including a known reverse control channel thereof;
digitizing the received wideband spectrum signals;
storing the digitized signals as digitized data in a delay memory;
monitoring the known reverse control channel signal for a cell phone transmission including a predetermined number of either a cell phone to be located or a phone number being called;
identifying a predetermined number from the monitoring of the reverse control channel; and storing data from the delay memory in response to the identifying of the predetermined number, including the portion of the received signal bearing identifying data of the predetermined number.
receiving signals transmitted over all channels of a wideband, cell phone base station operating spectrum including a known reverse control channel thereof;
digitizing the received wideband spectrum signals;
storing the digitized signals as digitized data in a delay memory;
monitoring the known reverse control channel signal for a cell phone transmission including a predetermined number of either a cell phone to be located or a phone number being called;
identifying a predetermined number from the monitoring of the reverse control channel; and storing data from the delay memory in response to the identifying of the predetermined number, including the portion of the received signal bearing identifying data of the predetermined number.
10. The method of claim 9, wherein the steps of base signals, digitizing received signals, storing digitized signals, and storing data from the delay memory are performed at a plurality of additional base stations, and further wherein the step of storing data from the delay memory at the plurality of addition base stations is performed in response to the step of identifying a predetermined number performed at the first base station.
11. The method of claim 10, wherein the storing of data from the delay memory includes digitally filtering data stored in the delay memory to retain data from only a single selectable cell phone channel.
12. The method of claim 11, further comprising the steps of generating a sampling signal for the digitizing of the received signals along with corresponding time tag signals for the stored digitized data, and storing the corresponding time tag signals in the delay memory with the digitized data.
13. The method of claim 12, further comprising the steps of determining reverse voice channel assignments and storing data from one or more reverse voice channel assignments by digitally filtering data from the delay memory to retain only digitized reverse voice channel data and its corresponding time tag.
14. An apparatus for collecting transmission signals at a first cell phone base station from a particular cell phone for the purpose of determining location of the cell phone using time difference of arrival, tdoa, comprising the steps of:
means for receiving signals transmitted over all channels of a wideband, cell phone base station, operating spectrum including a known reverse control channel thereof;
means for digitizing the received wideband spectrum signals;
means for storing the digitized signals as digitized data in a delay memory;
means for monitoring the known reverse control channel signal for a cell phone transmission including a predetermined number of either a cell phone to be located or a phone number being called;
means for identifying the predetermined number from the monitoring of the reverse control channel; and means for storing data from the delay memory in response to the identifying of the predetermined number, including the portion of the received signal bearing the identifying data of the predetermined number.
means for receiving signals transmitted over all channels of a wideband, cell phone base station, operating spectrum including a known reverse control channel thereof;
means for digitizing the received wideband spectrum signals;
means for storing the digitized signals as digitized data in a delay memory;
means for monitoring the known reverse control channel signal for a cell phone transmission including a predetermined number of either a cell phone to be located or a phone number being called;
means for identifying the predetermined number from the monitoring of the reverse control channel; and means for storing data from the delay memory in response to the identifying of the predetermined number, including the portion of the received signal bearing the identifying data of the predetermined number.
15. The method of claim 14, wherein the means for storing data from the delay memory includes means for digitally filtering data stored in the delay memory for retaining data from only a single selectable cell phone channel.
16. The method of claim 15, further comprising means for generating a sampling signal for the means for digitizing along with corresponding time tag signals for the stored digitized data, and means for storing the corresponding time tag signals in the delay memory with the digitized data.
17. The method of claim 16, further comprising means for determining reverse voice channel assignments and means for enabling the storing of data from one or more reverse voice channels assignments by controlling the means for digitally filtering data from the delay memory to retain only digitized reverse voice channel data and its corresponding time tag.
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CA1122686A (en) * | 1977-05-31 | 1982-04-27 | David W. Wind | Locating device |
US5317323A (en) * | 1993-03-05 | 1994-05-31 | E-Systems, Inc. | Passive high accuracy geolocation system and method |
US5327144A (en) * | 1993-05-07 | 1994-07-05 | Associated Rt, Inc. | Cellular telephone location system |
-
1994
- 1994-07-08 US US08/272,725 patent/US5512908A/en not_active Expired - Lifetime
-
1995
- 1995-07-07 NZ NZ289718A patent/NZ289718A/en unknown
- 1995-07-07 CN CN95190752A patent/CN1134752A/en active Pending
- 1995-07-07 KR KR1019960701287A patent/KR960705223A/en not_active Application Discontinuation
- 1995-07-07 EP EP95925549A patent/EP0720749A1/en not_active Withdrawn
- 1995-07-07 AU AU29645/95A patent/AU2964595A/en not_active Abandoned
- 1995-07-07 WO PCT/US1995/008526 patent/WO1996002006A1/en not_active Application Discontinuation
- 1995-07-07 CA CA002171325A patent/CA2171325A1/en not_active Abandoned
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EP0720749A1 (en) | 1996-07-10 |
US5512908A (en) | 1996-04-30 |
WO1996002006A1 (en) | 1996-01-25 |
AU2964595A (en) | 1996-02-09 |
NZ289718A (en) | 1998-08-26 |
KR960705223A (en) | 1996-10-09 |
CN1134752A (en) | 1996-10-30 |
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FZDE | Discontinued | ||
FZDE | Discontinued |
Effective date: 19980707 |